The EF 70-200mm f/2.8 L is THE zoom telephoto lens to own if you
are a serious Canon shooter. Its optical performance parallels that of
prime lenses, and it offers incredible performance (resolution, contrast
and bokeh) and versatility under most shooting conditions where a
telephoto lens is needed.

I tested the IS version of lens against the non-IS version of the lens
in 2004 initially and have updated the review in 2007 with newer digital
camera performance results.

Strong points of both the
IS and non-IS lens:

High contrast, high resolution images over a moderatly wide
telephoto range

Resolution Performance Data - The non-IS
lens outperforms the IS lens

The non-IS lens consistently outperformed the IS lens in resolution at
70, 85, 135 and 200mm focal lengths. I was initially surprised by the
lower resolution of the image stabilized lens. However, after reading more
about the complex construction of the image stabilized lens, it is not
terribly surprising that an IS lens with 23 glass elements in 18 groups is
not quite as sharp as a superbly sharp non-IS lens with 18 glass elements
in 15 groups.

The non-IS lens gives slightly better
performance than the IS lens, especially when shooting at f/2.8.

Detectable Sharpness Differences in Film Images and Digital Images

I tested whether the resolution difference was detectable in digital or
film images using an EOS-1Ds camera and an EOS-1n camera with Fuji NPH
film and the lenses set at 135mm. Details
of the test method are here.

Image Stabilization Benefits - when you find yourself in low
ambient lighting without a tripod.

Image stabilization works as advertised for hand
held work. A photographic series taken handheld at 200mm under a
constant light source demonstrated that I could shoot as slow as 1/15th
second and get sharp images with the IS lens. 1/60th of a second was as
slow as I could shoot consistently with the non-IS lens and get sharp
images.

Conclusion

For most photographic applications, either 70-200mm f/2.8 lens is an
outstanding tool that produces exceptional image quality. Deciding whether
to buy the IS lens or the non-IS lens should probably revolve around the
photographer's perceived need to have image stabilization. Image
stabilization does come at the cost of higher purhase price and slight
loss of image sharpness (You won't notice it in most situations unless you
shoot predominantly at f/2.8).

If you photograph stationary subjects in low light without a tripod, the
decision to purchase the IS version of the lens is easy.